Method of matching optical elements and fiber ferrules

ABSTRACT

An apparatus and method of building multiple-port optical devices characterizes optical filters according to a desired angle of incidence. Fiber ferrules are manufactured to precisely position at least two pairs of optical fibers inside the ferrules. The fiber ferrules are then characterized according to the distance between the fiber cores of the fiber pairs (i.e. the separation distance). The filters and the fiber ferrules are matched according to predetermined tolerances. The filter, or other optical element, is optically aligned with each pair of optical fibers and bonded into place. Light signals, such as DWDM signals, are then transmitted through the devices and the single filter or optical device operates on the multiple signals.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of co-pending U.S.patent application Ser. No. 09/599,168, filed on Jun. 22, 2000, entitled“THREE-PORT FILTER AND METHOD OF MANUFACTURE,” by Scott M. Hellman etal., the entire disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to optical telecommunicationsystems and, in particular, to an apparatus and method of manufacturingoptical devices employed in such telecommunication systems.

[0004] 2. Technical Background

[0005] Up to three port filtering and isolating packages are widely usedin local and long distance optical telecommunication networks. Thesenetworks comprise various spectral shaping and isolating opticalassemblies as parts of dense wavelength division multiplexing (DWDM)systems. The necessity to design reliable optical devices for suchsystems, which are subject to various thermal and mechanical loadsduring their 20 to 25 year lifetime, is of significant importance. Atypical example of such optical devices is an optical filter assembly. Atypical optical filter assembly comprises two (input and reflective)optical glass fibers inserted into a dual-capillary ferrule to produce afiber-ferrule sub-assembly, a GRIN lens, and a filter. The opticalcomponents of the filter assembly are embedded into an insulating glasstube, which in turn is mechanically protected by a metal housing. In atypical 3-port package the above dual-fiber filtering assembly iscombined with an output collimating assembly leading to a single opticalfiber. These filter assemblies typically exhibit insertion losses higherthan desired, resulting in degraded overall performance of thecommunications system or module. The problem is particularly acuteduring exposure to ambient operating conditions where temperature isvariable.

[0006] Typical input glass ferrules employ one of two designs. A singlecapillary suitable for containing multiple glass fibers or separatecircular capillaries for each fiber have been used, each with relativelyshort (0.7-1.2 mm) fiber-receiving conical lead-in ends. With such inputferrules, the optical fiber is subjected to an S-bending over the shortconical end portion which typically exceeds 50% of the fiber diameter(for a fiber having a 125 μm diameter) on a span of about 6 to 10diameters in length. This excessive micro bending increases theinsertion losses. Although the multi-capillary design reduces thelateral deflection of fiber interconnects compared to the ellipticalsingle-capillary design, the short length of the cone end of suchferrules cannot reduce the micro bending of the fiber and its inherentinsertion loss. Fiber-ferrule subassemblies employing such ferrules aremanufactured by inserting the optical fibers stripped of their polymercoating into the respective ferrule capillaries; epoxy bonding thefibers into the ferrule capillaries, including the conical end portions;grinding and polishing an angled facet on the fiber-ferrule; anddepositing on the polished surface an anti-reflection (AR) coating. Oncefinished, the fiber-ferrule is aligned and assembled with thecollimating GRIN lens and then embedded into the insulating glass tube,which, in turn, is protected by a metal housing.

[0007] There are two different technical solutions used in the design ofbonds securing the components of an optical assembly. A low compliancebond between thermally well matched glass fibers and the glass ferruleis an approach commonly used by some manufacturers. The adhesives usedare heat-curable epoxies with high Young's modulus (E>100,000 psi) andmoderate to high thermal expansion coefficients ((α=40-60 10⁻⁶° C.⁻¹). Atypical example would be 353 ND EPO-TEK epoxy adhesive. In addition, thebond thickness used is very small.

[0008] Silicon adhesives are used to bond thermally mismatched glasstubes with metal housings and glass optical elements with metal holders.In these joints, a high compliance design is used. The silicones, whichcan be cured between 20-150° C. in the presence of moisture, aretypically characterized by an extremely low Young's modulus (E<500 psi)and high thermal expansion (α=180-250 10⁻⁶° C.⁻¹). A typical examplewould be DC 577 silicone, which can be used to bond, for example, ametal optical filter holder to a GRIN lens.

[0009] Adhesive bonding with subsequent soldering or welding is used toencapsulate a filtering assembly into a three-port package of a DWDMmodule. A precise alignment achieved during initial assembly of a filterprior to final packaging can be easily decreased due to the adhesivecuring process and the high temperature thermal cycles associated withsoldering or welding during the final packaging of the components. Suchmanufacturing processes and resulting components have several problemsresulting from stresses on the optical components due to the thermalcontraction mismatch between the glass and metal materials,polymerization shrinkage in adhesive bonds, and structural constraintsinduced by bonding and final soldering during encapsulation. Thesestresses lead to displacements of optical components during bonding andsoldering, resulting in 0.3 to 1 dB or greater increases in theinsertion loss.

[0010] Such a filter package enclosure, which is typically formed of sixto eight concentric protective units, has micron transverse tolerances.Maintaining these tolerances requires precision machining,time-consuming alignment, and soldering with frequent rework. As aresult of these limitations, the optical performance specifications arelowered and cost is increased. As an example, soldering typicallyincludes several re-flow cycles. This induces local thermal stresses inthe nearby adhesive bonds and leads to the degradation of the polymeradhesive, resulting in repositioning of optical components and a shiftin the filter spectral performance. With such design, soldering may alsoresult in the contamination of optical components through direct contactwith molten solder and/or flux.

[0011] However, for many applications, it is desirable to obtain a highaccuracy thermally compensated optical multiple-port package that can berelatively inexpensive, reliable, and have a low insertion loss.Additionally, a package design should be adequate not only tomechanically protect the fragile optical components but also tocompensate for and minimize the thermally induced shift in spectralperformance. Further, it is desirable to obtain a multiple-port package,such as six port packages, with the same qualities since they furtherreduce costs, reduce size, and also result in reduced insertion loss.Thus, there exists a need for such optical packages and a process formanufacturing such optical packages, which is miniaturized, has a lowinsertion loss, is inexpensive to manufacture, and which results in adevice having reliable, long-term operation.

SUMMARY OF THE INVENTION

[0012] The present invention provides an improved optical assembly(e.g., optical filter assembly) with a low insertion loss (IL) andprovides an assembly of the optical components, such as input ferrules,collimating lenses, and filters, utilizing bonding adhesives in a mannerwhich allows the alignment of the individual components relative to oneanother with a precision and a manufacturability that makes it possibleto produce commercial devices having five, six or more ports. This hadheretofore not been achieved. In one aspect, the invention includes animproved input ferrule and filter holder which permits active alignmentand bonding through the utilization of UV and thermally curableadhesives and improved thermal curing to greatly reduce relevantinternal stresses in the subassembly so formed. For assemblies havingmultiple pairs of fibers (e.g., five or more port devices) the inventionalso provides improved fiber ferrule designs, alignment methods, andmethods to permit the manufacture of devices that have low IL, operateover a wide temperature range, are reliable, and cost effective.

[0013] In one aspect of the invention, improvements to fiber ferrulesare provided including capillary designs and tolerances. The inventionprovides designs for capillaries which resist movement of the opticalfibers during adhesive curing, soldering, welding, and environmentalthermal changes. One technique uses washers to precisely positionoptical fibers in a capillary. Yet another aspect of the invention isthe selection of optical fibers based on geometric properties such as:outer (cladding) diameter, circularity of the cladding (ovality), andcore concentricity. In another aspect, the invention teaches matchingthe separation distance (SD) between optical fibers and the relationshipto angle of incidence (AOI) of the optical filter. Tolerances for theseparation distance are provided which make possible the commercialmanufacturability of multiple-port devices with five, six or more ports.The optical alignment process becomes more critical and complex as thenumber of ports increases and therefore the invention provides methodsfor handling this more complex alignment. A method of selecting anoutput collimating assembly is also provided.

[0014] Methods embodying the present invention include the steps ofactively aligning a filter holder and filter to a collimator assemblyincluding a GRIN, aspheric, or other collimating lens mounted thereto,axially separating the filter holder and lens in a movable fixture,placing a UV and thermally curable adhesive on the periphery of thelens, moving the lens into engagement with the filter holder having afilter mounted therein, aligning the collimator assembly with respect tothe filter holder while monitoring the input and reflected signals ofthe optical fibers coupled to the lens for insertion loss less thanabout 0.2 dB, and applying UV radiation through the filter end of thefilter holder to initially cure the aligned subassembly. In anembodiment of the invention, the subassembly is subsequently thermallycured through an accelerated dark cure sequence followed by a final hightemperature curing. In another embodiment of the invention, UV radiationis applied to the filter holder/lens interface through one or moreapertures formed in the side of the filter holder which overlaps thelens. The UV light source may be dithered such that UV radiationuniformly covers the cylindrical interface between the filter holder andthe outer surface of the lens. In yet another embodiment of theinvention, the filter and lens are pre-aligned prior to the applicationof adhesive by monitoring the input and reflected signals of the fiberswhile adjusting the X-Y positioning for a maximum detected signal.

[0015] In a preferred method of manufacturing the invention, subsequentto the UV curing process, the assembly is cured through a stressrelaxation cycle at about 40-50° C. for two to four hours followed by athermal curing cycle of about 95 to 110° C. for one to two hours.

[0016] In one embodiment of the invention, an input ferrule is employedwith an input cone having an axial length greater than about 2.5 mm toreduce S-bending of input fiber, thereby minimizing resultant insertionlosses. In another embodiment of the invention, a generally cylindricalfilter holder having an annular seat formed in one end for receiving afilter and a lens-receiving aperture at an opposite end having aninternal dimension which allows micro-tilting of the filter holderrelative to the lens is used to provide an alignment of the filter at anangle of less than about 1° to the axis of the lens. The preferredfilter holder includes slots or openings in the lateral surface suchthat UV light enters and cures adhesive between the lens and filterholder. An optical filter assembly of a preferred embodiment of thepresent invention includes such an improved ferrule and/or a filterholder coupled in alignment with one another in a suitable housing.

[0017] The methods and apparatus described herein facilitate themanufacture of a multiple-port optical device which results in severaladvantages. For example, in a six port device having two pair of opticalfibers in the input collimating assembly, one filter operates with atleast two transmitted light beams and splits the beams into at least tworeflected and two transmitted beams, thereby reducing by half the numberof optical filters, collimating lenses and enclosure units. Thus, forexample, the same six-port filtering package can be used in themultiplexing and de-multiplexing operations of a DWDM moduleincorporating concatenated six-port packages. A typical DWDM moduleincludes from two to eight six-port packages. In this case, the numberof filter chips, collimating lenses, and fiber ferrules will be reducedby one half compared to using three port packages.

[0018] The manufacturing method and optical element assembly of thepresent invention, therefore, provides an improved performance opticalassembly utilizing a unique input ferrule, filter holder, and anassembly method for providing a low cost, highly reliable, and improvedperformance optical element assembly, such as a three-port collimatingfilter or six-port collimating filter assemblies, and using theseassemblies in DWDM modules which can be used in an opticalcommunications system.

[0019] The devices of the instant invention are applicable for bothsingle mode optical fibers that are applicable to DWDM operations andfor polarization maintaining fibers that can be used in the crystalbased isolators, circulators, polarization splitters, and the like.

[0020] Additional features and advantages of the invention will be setforth in the detailed description which follows and will be apparent tothose skilled in the art from the description or recognized bypracticing the invention as described in the description which followstogether with the claims and appended drawings.

[0021] It is to be understood that the foregoing description isexemplary of the invention only and is intended to provide an overviewfor the understanding of the nature and character of the invention as itis defined by the claims. The accompanying drawings are included toprovide a further understanding of the invention and are incorporatedand constitute part of this specification. The drawings illustratevarious features and embodiments of the invention which, together withtheir description serve to explain the principals and operation of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a perspective view of a filter subassembly embodying thepresent invention;

[0023]FIG. 2 is a partial vertical cross-sectional schematic view of thesubassembly shown in FIG. 1;

[0024]FIG. 3 is a vertical cross-sectional schematic view of athree-port filter assembly embodying the present invention;

[0025]FIGS. 4 and 4A are enlarged vertical cross-sectional and right endview, respectively, of a prior art ferrule employed in a prior artfilter assembly;

[0026]FIGS. 5 and 5A are an enlarged vertical cross-sectional view andright end view, respectively, of a ferrule employed in the filtersubassembly of FIGS. 1 and 2 and the filter of FIG. 3;

[0027]FIG. 6 is an enlarged vertical cross-sectional schematic view ofan improved filter holder of the present invention also illustrating itsmethod of assembly;

[0028]FIG. 7 is a schematic view illustrating the frontal polymerizationof a UV or thermally curable bonding adhesive when UV light ispropagated transversely through a filter, as illustrated in FIG. 6;

[0029]FIG. 8 illustrates the spectrum of a mercury light source showinga significant portion of the UV light spectrum;

[0030]FIG. 9 is the measured UV-transmission spectrum of a commerciallyavailable thin film filter used in the structure shown in FIGS. 1, 2, 3,and 6;

[0031]FIG. 10 is a graph of the accelerated dark cure and thermal cureof the subassembly shown in FIG. 6;

[0032]FIG. 11 is a perspective view of an alternative embodiment of afilter holder embodying one aspect of the present invention;

[0033]FIG. 12 is a vertical cross-sectional schematic view of athree-port filter employing the filter holder shown in FIG. 11;

[0034]FIG. 13A is a cross-sectional view of a fiber-ferrule assemblyillustrating a rounded square capillary;

[0035]FIG. 13B is a cross-sectional view of a fiber-ferrule assemblyillustrating a dual-oval capillary;

[0036]FIG. 13C is a cross-sectional view of a fiber-ferrule assemblyillustrating a four-circular capillary;

[0037]FIG. 13D is a cross-sectional view of a six-fiber ferrule having arectangular capillary;

[0038]FIG. 13E is a cross-sectional view of a fiber-ferrule assemblyillustrating capillaries formed by symmetrical grooves formed in dualsilicon wafers;

[0039]FIG. 13F is another embodiment of a fiber-ferrule formed from twowafers;

[0040]FIG. 13G is a cross-sectional schematic view of a finishedtwo-wafer ferrule inside a glass sleeve;

[0041]FIG. 13H illustrates the preferred V-groove and alignment rodconfiguration;

[0042]FIG. 13I is a cross-sectional view of a fiber ferrule illustratingalignment of fibers with two wafers and application of liquid adhesive;

[0043]FIG. 13J is a cross-sectional view of a fiber ferrule having arectangular capillary for variable separation distance;

[0044]FIG. 13K is a cross-sectional view of a fiber ferrule having dualrectangular capillaries for variable separation distance;

[0045]FIG. 13L is a cross-sectional view of a fiber ferrule havingelongated dual rectangular capillaries;

[0046]FIG. 13M is a cross-sectional view of a fiber ferrule having dualoval capillaries;

[0047]FIG. 13N is a cross-sectional view of a fiber ferrule having threecapillaries;

[0048]FIG. 14A is a view of an alignment washer;

[0049]FIG. 14B is a cross-sectional exploded view of a fiber-ferruleassembly using alignment washers; and

[0050]FIG. 15 is an exemplary table for matching optical fiber SD andfilter AOI.

[0051]FIG. 16A is a schematic diagram of a four-port filter assembly;

[0052]FIG. 16B is a schematic diagram of a four-port filter assemblycoupled to an amplifier;

[0053]FIG. 16C is a schematic diagram of a four-port filter assemblycoupled to two amplifiers;

[0054]FIG. 16D is a schematic diagram of five-port filter package;

[0055]FIG. 16E is a schematic diagram of a six-port filter packagecoupled to waste energy terminals;

[0056]FIG. 16F is an alternate schematic diagram of a six-port filterpackage coupled to waste energy terminals;

[0057]FIG. 16G is a schematic diagram of a six-port add/drop package;

[0058]FIG. 16H is a schematic diagram of an eight-port optical package;

[0059]FIG. 16I is a schematic diagram of an eight-port add/drop package;

[0060]FIG. 17 is a schematic diagram of concatenated six-port packagesto form a DWDM module.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0061] Referring initially to FIGS. 1 and 2, a brief description of anoptical element (e.g., filter) subassembly 10 is first presented. Theinvention is described and illustrated using an exemplary three-portfilter device, however, the invention also applies to multiple-portdevices such as six-port devices. For multiple-port devices the numberand position of fibers in ferrule 16 changes accordingly. The dual fibercollimating and filtering subassembly 10 includes an outer cylindricalmetal housing 12, which is bonded at 13 (FIG. 1) around input andreflection optical fibers 18 and 20 respectively. Housing 12 surroundsan insulating cylindrical boro-silicate or fused silica sleeve 14 (FIG.2) within which there is mounted a dual capillary glass ferrule 16receiving an input optical fiber 18 and a reflective optical fiber 20.The ends of fibers 18 and 20 in ferrule 16 face a collimating lens 22,such as, for example, a GRIN lens, which has polished facets on theinput end, and (as seen in FIG. 2) which face and align with the ends ofoptical fibers 18 and 20 held in place by ferrule 16. Lens 22 collimateslight from input fiber 18 into parallel rays, transmitting them to anoptical element which may be a thin film filter 24, a birefringentcrystal, or other appropriate optical element. The end of thecollimating lens 22 that is closest to the filter 24 is referred to asthe output end of collimating lens 22. A filter holder 26 is mounted tothe end 21 of the collimating lens 22 according to the method of thepresent invention and includes an axial aperture 27 allowing light fromlens 22 to impinge upon filter 24 and the reflective light to bedirected to reflective optical fiber 20. Filter holder 26 also securesfilter or crystal 24 in alignment with the collimating lens 22 withaperture 27 extending between the filter 24 and lens 22. Thefiber-ferrule 16, lens 22, and insulating sleeve 14 are collectivelyreferred to as an input collimating assembly 35. Collimating assembly 35may also include cylindrical metal housing 12. A similar single fibercollimating assembly structure is collectively referred to as an outputcollimating assembly 35′ and is shown in FIG. 3.

[0062] Before describing the manufacture of the subassembly 10 forming apart of an overall three-port filter, a three-port filter 30 is brieflydescribed. FIG. 3 is also representative of a multiple-port device,however, for a multiple-port device the number and position of fibers inferrules 16 and 39 change accordingly. As shown in FIG. 3, three-portfilter 30 includes an outer cylindrical metal sleeve 32 into whichsubassembly 10 is mounted and secured by a cylindrical interface ofsolder and/or welding material 31 applied to the solder joint as seen inschematic diagram of FIG. 3. Solder and/or weld material 31 may beapplied through suitable apertures 32A in metal sleeve 32. The outputsignal from filter 24 is received by an aligned collimating output lens34 similarly secured within a boro-silicate or fused silica glass sleeve36 surrounded by a metal sleeve 37 which, in turn, is mounted within theinterior of outer protective sleeve 32 utilizing a cylindrical solderinterface 33. The output lens 34, ferrule 39, glass sleeve 36, and metalsleeve 37 form the output collimating assembly 35′. An output opticalfiber 38 couples to the desired wavelength output signal from three-portfilter 30 to the communication link in which the three-port filter 30 isinstalled. Thus, for example, the three-port filter 30 may be employedto receive a plurality of wavelengths from input optical fiber 18, passa single output wavelength to output fiber 38, and return the remainingsignal wavelengths to reflective optical fiber 20. The method ofassembling subassembly 10 and its structural elements, are unique and isdescribed in detail below. Further, the specific method of aligningoutput collimating assembly 35′ within sleeve 32 will also be describedbelow.

[0063] One problem associated with prior art ferrules is illustrated byFIG. 4 showing a vertical schematic cross-sectional view of a prior artinput ferrule 40. Ferrule 40 is made of a conventional glass materialsuch as fused silica or boro-silicate glass and includes a pair ofspaced-apart capillaries 42 and 44 having a diameter sufficient toreceive the stripped input and reflective optical fibers 18 and 20having a diameter of about 125 μm. The overall diameter, however, ofoptical fibers 18 and 20 includes a protective polymeric sheath and isapproximately 250 μm. Optical fibers 18 and 20 are cemented within theconical input section 46 of the prior art ferrule 40 utilizing athermally curable epoxy adhesive providing a strain-relief connection ofthe coated fibers 18 and 20 within the glass ferrule. As the strippedoptical fibers 18 and 20 exit the polymeric sheath and enter thecapillary tubes 42 and 44 over the length of 1.2 mm of the conical inputsection 46, they are bent at area 47 schematically shown in FIG. 4. ThisS-bending of the optical fibers interconnection to the ferrule 40results in deflection of the fiber, which exceeds 50% of the fiberdiameter. This induced micro bending of the fiber increases insertionloss of the signals applied to the lens 22 due to the geometry offerrule 40.

[0064] Capillaries 42 and 44 of ferrule 40 are spaced apart a distance“D1”, as shown in FIG. 4A that with the coned length provided by priorart ferrules as shown in FIG. 4, results in such excessive micro-bendingof the optical fibers and resultant insertion losses. The alternateferrule construction in which a single elliptical capillary is providedfor receiving adjacent optical fibers and having a similar input coneconstruction suffers even more from the bending problem. In order togreatly reduce the insertion loss due to the undesirable S-bending ofinput fibers, an improved ferrule 16 of the present invention, whichforms part of the subassembly 10 as seen in FIGS. 1 and 2, is employedand is described in FIGS. 5 and 5A.

[0065] In FIG. 5, a ferrule 16 is shown which has an input cone 17 withan axial length in the preferred embodiment in excess of 2 mm andpreferably 2.4 mm or approximately twice the length of prior art inputcones. The input diameter “D2” of input cone 17 is approximately 0.8 mmto accommodate the 500 mm combined diameter of input fibers 18 andreflective fiber 20 and allow room for epoxy to bond the fibers withincone 17. The exit diameter “D3” of cone 17 adjacent capillaries 19 and21, which receive and secure the optical fibers 18 and 20 therein, ispreferably determined as:

D3−2f_(d)+D1

[0066] or

D3=250 μm+D1

[0067] where f_(d) is the fiber diameter with the sheath materialremoved

[0068] This accommodates any spacing D1 between the fibers and the 125μm diameters of each of the stripped input and reflective fibers,allowing also approximately a one μm gap at the input to capillary tubes19 and 21 for epoxy to securely seat the input and reflective fiberswithin ferrule 16. To obtain the best possible performance, the fibersshould be selected for their geometric properties. Three importantproperties and the preferred tolerances are outer cladding diameter of125 μm +/−0.2 μm, non-circularity of the cladding less than 0.2%, andcore to cladding concentricity is less than 0.2 μm. By expanding theaxial length “L” of cone 17 to nearly twice that of prior art inputferrules, S-bending is substantially avoided, providing substantially anearly equal optical path length for both the input and reflectivefibers and reducing insertion losses. This technique is also applicableto ferrules having more than two optical fibers and to ferrules withsingle or multiple capillaries.

[0069] The fibers are epoxied within the ferrule 16 with an epoxyadhesive such as, for example, 353 ND EPO-TEK epoxy adhesive availablefrom Epoxy Technology, Billerica, Mass., and cured at about 110° C. forone and one-half hours. It is preferable to post-cure the assembly at125-130° C. for one-half hour to reduce moisture absorption. Theend-face 28 of the ferrule with inserted and bonded optical fibers areground and polished to produce approximately 8° angle elliptical facetto the axis of the ferrule. Ferrule 16 is then cemented within thesurrounding thermally insulating glass sleeve 14 (FIG. 2) to form inputcollimating assembly 35. Prior to the insertion of the ferrule 16 intosleeve 14, the lens 22 has been installed and cemented in place. Theferrule is aligned with a gap “G” (FIG. 2) of about 1 to 1.5 μm betweenthe ends of the lens 22 and the ferrule to allow the axial androtational active alignment of the ferrule to the lens 22 by rotatingthe ferrule within sleeve 14 and axially positioning it to accommodatethe surface angle of the lens 22, which may run between 7.8° to 8.1°.For a three-port assembly, a signal is applied to the input fiber 18while monitoring the output of the GRIN lens within sleeve 14. For amultiple-port assembly, such as used in a six-port device, the alignmentprocess is similar, however signals are applied to each of the inputfibers and the ferrule is axially and rotationally positioned tooptimize the alignment for all of the signals. This assures the minimuminsertion loss and maximum signal coupling between the optical fibersand the input collimating lens 22, which subsequently receives thefilter holder and filter therein as now described in connection withFIG. 6.

[0070] Referring now to FIG. 6, the subsequent positioning of filter 24and filter holder 26 onto end 21 of the lens 22 is described. Matchingthe AOI of the filter 24 with the separation distance (SD) of the fibers18 and 20 is important. A filter 24 with a desired AOI is selected foruse in the assembly 10. An input collimating assembly 10 is selectedhaving a ferrule 16 which has an SD that corresponds to the AOI offilter 24. The SD is accurately measured, preferably within 0.5 μm, andthe filter holder 26 is mounted on the selected input collimatorassembly 35. The matching process in the case of the four-fiber ferrule,used in six-port devices, is preferably performed as follows. The SD ofone of the pairs of fibers is matched to the filter AOI. The alignmentmatch for the second pair of fibers is provided automatically when thestructural tolerances described above for the capillaries and fibershave been satisfied. Therefore it is important for the SD for each pairof fibers to be approximately equal. Preferably the SD tolerance foreach pair of fibers is within 0.5 μm. The tolerances are furtherdiscussed below in discussion of FIGS. 14 and 15.

[0071] Filter holder 26 has a cylindrical aperture 25 at its lower end,as seen in FIG. 6, which overlies the cylindrical diameter of lens 22.The diameter of the aperture 25 is large enough to provide a gap “G1” ofabout 50 μm surrounding output end 21 of collimating lens 22. This, asdescribed below, allows the micro-tilting of the filter holder 26 withrespect to lens 22 for precisely aligning the filter 24 and lens 22while accommodating the bonding adhesive employed for securing thefilter holder to the lens 22. Holder 26 is made of a material which hasa coefficient of thermal expansion which is close to that of the lensand, in a preferred embodiment of the invention, is a unit made of SSI7-4-PH stainless steel. Prior to assembling of filter holder 26 to lens22, the filter 24 is mounted within the filter holder 26, which has acylindrical aperture 29 with a seat 50 canted at an angle ocx (FIG. 6)of approximately 1.50 to 2° and preferably about 1.8° to accommodate theapproximate 0.3° to 0.7° angular discrepancy between the front and rearsurfaces of a typical filter chip 24. The cant of seat 50 also has thefavorable effect of reducing the tilt angle of holder 26 relative tolens 22. The filter 24 is secured within cylindrical aperture 29utilizing conventional epoxy or even silicone bonding adhesives, such asDC577 or CV3 2000, and the filter chip 24 can be any commerciallyavailable thin-film filter. In the illustrated embodiment, acommercially available filter having dimension of, for example, 1.4 by1.4 by 1.5 mm is used. Such filters are available commercially fromComing Incorporated. The assembly and met hods of the invention can alsobe used with other optical devices in place of filter 24, such asvarious crystal-based components.

[0072] With filter 24 in place in filter holder 26, the holder isclamped in a vertically (as seen in FIG. 6) movable clamp which can alsobe rotated such that filter holder 26 can be moved into and out ofengagement with lens 22 as well as rotated and tilted for activelyaligning the optical axis of the filter to the lateral surface of thelens 22 to minimize insertion loss. Active alignment is the process ofaligning the optical elements while applying input light signals to thedevice and monitoring an output signal. This is in contrast to passivealignment which is the process of aligning optical elements in theabsence of a light signal.

[0073] The active alignment in an embodiment of the invention isachieved, for example, by applying a signal at about 1530 nm to inputfiber 18 (FIGS. 1-3) while monitoring the reflected signal on fiber 20.Filter holder 26 is then micro-tilted in orthogonal directions and alsorotated in increments of about 2° to 5° as necessary to achieve minimuminsertion loss as determined by monitoring the input and reflectedsignals. There are six degrees of freedom in which the holder 26 may bemoved relative to lens 22. These include micro-tilting on the XZ planeand YZ plane of FIG. 6, rotating about the Z axis, moving lateral alongthe X and Y axis, and raising and lowering the holder along the Z axis.Generally, only rotation and micro tilting along the XZ and YZ plane aresufficient to align the elements.

[0074] The preferred embodiment uses an automated iterative process inwhich the IL for each pair of fibers is monitored for each tilt orrotation. The iterative process repetitively adjusts the filter holderand monitors the input and output signals and eventually locates anoptimum alignment as defined by predetermined tolerances. The alignmentprocess increases in complexity with increasing pairs of optical fibersin multiple-port systems. The preferred method of alignment comprisesthe steps of aligning each pair of fibers separately and then selectingan average alignment position or a median position. For six-portdevices, the optimum alignment achieved for the first pair of thereflective and input fibers can be slightly lowered when aligning thesecond pair of the reflective and input fibers. Also, in the case ofsix-port devices, the iterative process has been found to beunexpectedly short (i.e. few iterations) because of the tolerancesselected in accordance with an embodiment of the invention. When a firstpair of fibers is optically aligned, the second pair of fibers may beclose to alignment since the second pair of fibers have virtually thesame separation distance as the first pair of fibers.

[0075] During this alignment process, lens 22 and its sleeve 12 aremounted in an XYZ micro-adjustable stage of conventional construction tohold the projecting end of lens 22 in cavity 25 of holder 26. Once theoptimum angular position of the filter holder 26 to lens 22 isdetermined, the filter holder 26 is raised axially away from the lens(while maintaining the angular relationship) to allow access to the sidewall of lens 22. While separated, preferably four or more drops ofbonding adhesive is positioned on the outer peripheral circumferentialsurface of the end 21 of lens 22, with care being taken not to touchdrops of the epoxy adhesive to the lens end face surface. The filterholder 26 is then lowered over the lens 22, wiping the adhesive in theannular space between cavity 25 and lens 22. Next, the XZ axis of thestage may be further adjusted while monitoring signals applied to theinput and reflective optical fibers 18 and 20 to assure a minimuminsertion loss. Similarly, the YZ axis of the stage may also be adjustedwhile monitoring the signals to assure proper alignment and a minimumreflected insertion loss of no greater than about 0.3 dB. A variety ofUV and thermally curable epoxies were tested, and it was determined thatthe bonding adhesive which worked unexpectedly well was commerciallyavailable EMI-3410, which is a UV and thermally curable filled adhesiveavailable from Electronic Materials, Inc., of Breckenridge, Colo.

[0076] By providing a gap of approximately 50 μm between the innersurface of cylindrical aperture 25 of filter holder 26 and the outerdiameter of lens 22, the optical axis of the lens can be preciselyaligned with the optical axis of filter 24. Filter holder 26 beingadjustable within an angle ∝₂ of less than about 1.0°, as shown in FIG.6. This active alignment of the lens 22 and filter holder 26 is achievedby the movement of the lens 22 in the XZ and YZ planes, as shown in FIG.6, utilizing a standard micro-stage (i.e. micropositioner). In oneembodiment of the invention, one or more sources of ultra violetradiation such as sources 60 and 61 are employed to expose the bondingadhesive at the interface between holder 26 and lens 22 to ultravioletradiation to cure the bonding adhesive sufficiently such that thedesired relationship between the lens 22 and filter 24 is fixed untilthe adhesive is finally thermally cured.

[0077] As seen by the diagram of FIG. 7, by injecting ultra violetradiation from source 60 into the exposed end of filter 24, ultra violetradiation (indicated as 63) is dispersed as the UV radiation propagatestransversely through the filter and into the adhesive layer 55 (FIG. 6),causing frontal polymerization of the adhesive due to UV lightpropagating through the filter. In most instances, the UV radiation 63from source 60 through filter 24 will, upon an exposure of about 20seconds at a distance of about 2.5 cm between the source and the filter24 result in sufficient UV curing of the adhesive to fix the filterholder to the lens 22. In addition to exposing the adhesive 55 throughfilter 24 utilizing a UV light source 60, an additional UV light source61 can be employed to direct UV radiation 63 through the gap G2 betweenthe lower annular end of filter holder 26 and the top annular surface ofsleeve 12 with 40 second exposures for a total exposure of about 100seconds of UV radiation to cure the adhesive in the annular area of gapGi at the lower end of filter holder 26. After the UV curing, whichtends to temporarily induce stresses typically of from 200 to 300 psi orhigher in the subassembly, thermal cure stress release and curing isprovided as described below. Before such curing, however, input andoutput signals are monitored to assure that the reflected insertion loss(IL) remains less than about 0.3 dB and thermal change in IL is belowabout 0.05 dB. The UV from light source 61 can be rotated around theperiphery of the subassembly during successive exposures. The UV lightcan be delivered also through slots or openings formed into the lateralsides of the filter holder 22 as described below.

[0078] The UV sources 60 and 61 have spectral emissions, as illustratedin FIG. 8, which shows the spectrum of a mercury light source. FIG. 9illustrates the experimentally determined UV transmission spectrum ofsuch a light source through a bulk filter chip of the kind used in thefilter 24 illustrated FIG. 6. The convolution of these spectra indicatesthat a sufficient portion of the UV light spectrum propagates to thebond layer through the filter 24 and that the duration of the UV curecycle results in a nearly zero change of insertion loss over a periodfor from 630 to 700 seconds. The UV initiated cure induces initialstresses due to polymerization shrinkage. For a typically highly filledepoxy adhesive with a limited volume of shrinkage (on the order of0.2%), the induced stress can be as high as 300 to 600 psi. The stressesinduced by the UV curing, which fixes the alignment of the filter to thecollimating lens 22, are relieved and the bonding adhesive 55 furthercured during thermal curing of the subassembly 10 in a conventional ovenwhich is controlled to provide the stress relaxation and thermal curecycles as illustrated in FIG. 10.

[0079] The graph of FIG. 10 illustrates an accelerated and thermallyassisted stress relaxation phase in an oven which is controlled toprovide several short thermal cycles at an elevated temperaturepreferably not exceeding 50% of the minimum temperature of thermal cure.The cycle typically starts at room temperature, and the temperature isincreased to cycle between about 40° and about 60° C. over ten tofifteen cycles per hour for a total period of approximately one andone-half to four hours. The thermal cycling induces the variablemismatch stresses in the glass, metal filter holder, and the adhesive.Although the rate of stress relaxation in the adhesive increases with anincrease in the mismatch stresses, this stress level is limited by theallowable elastic limits. These cyclic changes in temperature induce thecreep in adhesive that leads to the additionally accelerated stressrelaxation. By cycling the temperature as shown in FIG. 10, thetypically 12 to 24 hour room temperature dark cure is reduced to aboutone to two hours. In this case, any thermally induced repositioning ofoptical components (e.g. filters) is drastically reduced.

[0080] As seen in FIG. 10 after the thermally assisted stress relaxationphase (TASR), the assembly is subjected to a final thermal cure forabout two to about two and one-half hours at a temperature of from about85° to about 100° in the case of the preferred EMI-3410 adhesive. Byutilizing the thermal curing cycle illustrated in FIG. 10, the elevatedtemperature induces a thermal mismatch stress in addition to theexisting shrinkage stresses. When the combined stresses are less thanthe isochronous elastic limit of the adhesive material, the acceleratedstress relaxation occurs with no irreversible deformation in the bond.This effect is substantially improved with increasing the number ofthermal cycles during the TASR phase (i. e., initial) portion of thethermal cure cycle.

[0081] Although the utilization of the UV light source 60 directingradiation 63 through filter 24 provides the desired initial UV curing ofthe adhesive bond between the filter holder and collimating lens, thefilter holder can be modified, as seen in FIGS. 11 and 12, to provideadditional axial exposure ports for exposure by UW radiation from radialsource 61 (as seen in FIG. 6) to improve the dispersion of UV radiationthrough the glass bonding adhesive layer 55.

[0082] As shown in FIG. 11, a filter holder 26′ is shown, which issubstantially identical to filter holder 26 with respect to theprovision of a cylindrical gap by its lower cylindrical aperture 25′ foradjustment of the filter holder to the lens; however, the lower end offilter holder 26′ includes a plurality of apertures such aslongitudinally extending, radially inwardly projecting slots 70 spacedaround the periphery of the filter holder and communicating withcylindrical opening 25′ within the filter holder 26′. Four to six slots70 have been found acceptable. Once a filter 24 is mounted in place asdescribed above in connection with filter holder 26, holder 26′ receivesepoxy as in the previously described embodiment, and the lens is raisedand adjusted with respect to filter 24 contained within filter holder26′ in the same manner as in the first embodiment. The light source 61,however, is moved around the periphery of the filter holder 26′directing UV radiation into slots 70 defining downwardly projecting,spaced apart legs 72 between such slots such that UV radiation isdithered into the cylindrical side walls of lens 22 which serves tofurther disperse the UV radiation uniformly within the annular spacecontaining bonding adhesive 55. By providing spaced radially extendingelongated slots 70 or other suitably shaped apertures extending throughthe side wall of the lower section of filter holder 26′ a light path isprovided for UV radiation to the inner cylindrical aperture 25′receiving the end of lens 22. In one embodiment, four slots 70 spaced at90° intervals around the lower section of holder 26′ were provided. Thisresults in improved uniform UV exposure to facilitate the UV curing ofadhesive 55. In this embodiment, it is unnecessary to expose the bondingadhesive utilizing a light source 60 through the filter since thebonding adhesive is uniformly exposed utilizing radiation from lightsource 61. Once the subassembly 10′, as shown in FIG. 12, is completed,it is assembled into the resultant three-port filter package 30′ in aconventional manner.

[0083] The above description is generally applicable to optical devicesranging from three-port devices to five-port devices, and to higher portdevices. The difficulty of manufacturing operational devices increaseswith the increased number of optical fibers and ports. Discussed beloware some of the features of the present invention which are directed todevices with five optical ports or more.

[0084] The uses and applications for five, six and higher port-countembodiments of the invention are many. For example, possibleconfigurations of multiple-port thin-film filters, splitters,circulators and isolators include: six-port devices that are formed fromtwo-fiber and four-fiber ferrule assemblies, eight-port devices that areformed from two four-fiber ferrule assemblies, and five-port devicesthat are formed from a single-fiber ferrule assembly and a four-fiberferrule assembly.

[0085] One important aspect of a multiple-port device is the tolerancefor the position of the optical fibers in the fiber ferrule 16. The coreof an optical fiber has a diameter of only about 9.5 μm. Consequently, a1 μm shift or error in the position of the fiber can cause the IL to beunacceptable. Therefore, great care must be taken to ensure the totaltolerance in the positioning of the fibers. To achieve these tolerances,the fibers should be pre-selected to provide the core concentricitywithin a tolerance of preferably about 1.0 μm, and more preferably about0.5 μm, and most preferably about 0.1 μm; cladding diameter of 125 μmwithin a tolerance of preferably about 1.0 μm, and more preferably about0.5 μm, and most preferably about 0.1 μm; and the ovality tolerance ofpreferably less than about 0.8%, and more preferably about 0.4%, andmost preferably about 0.12%. Concentricity is the deviation of thecenter of the optical fiber core from the center of the fiber. Ovalityis defined as the difference between the largest and smallest diameterof the fiber divided by the average diameter of the fiber (i.e. (D1−D2)*2 /(D1+D2) where D1 and D2 are the largest and smallest diameter of thefiber). The pre-screening and selection of the fibers for one or more ofthese characteristics has yielded the unexpected result of providing anassembly in which the fibers and other component parts can be assembledand aligned in a manner that can be reliably repeated and manufacturedfor commercial applications. Prior to the realization of this unexpectedresult, there were no commercially available optical packages havinggreater than three ports, and no commercially available six portpackages. Regarding ferrule capillary tolerances, the simplest “square”capillary ferrule is preferably characterized by a tolerance of theoutput end of the capillary of 252 μm+/−2 μm as the distance between twoparallel sides and more preferably 251 μm+/−1 μm and most preferably250.5 m +/−0.5 μm. Similar tolerances are preferred for the othercapillary shapes and configurations. Further, the tolerance of the fiberposition must be maintained throughout the manufacturing, packaging, andenvironmental conditions the device must endure. The methods andapparatus to achieve these tolerances are a subject of the presentinvention and are discussed below.

[0086] Although some prior art devices may initially achieve the desiredtolerances for the position of optical fibers, the prior art often failswhen the device is subject to stresses, strains and environmentalconditions that cause the fibers to shift sufficiently to exceed thetolerances. Causes of these stresses include: 1) viscous flow ofadhesive involving the fibers, 2) curing of the adhesives that bond thefibers to the ferrule and, 3) thermal stress due to the final packagingoperations or environmental testing conditions. During manufacture thedevices are subject to heat such as from solder used to encase thedevices in a protective metal sleeve 32. In use the devices are subjectto environmental conditions and must remain operational over aqualification temperature range from −40° C. to 85° C. (an industrystandard temperature range). Therefore, one aspect of the inventionrelates to a four-fiber-ferrule that satisfies the above mentionedtolerances.

[0087] Ferrules are generally cylindrical boro-silicate or fused silicacomponents with one, two, three or more capillaries for receiving theoptical fibers. Ferrules 16 were discussed above in discussion of FIGS.2 and 3, however, the capillaries for six-port devices are preferablydifferent. The shape of the drawn capillaries and the illustrativefabricating techniques allow fibers to be not only symmetricallyseparated from the central axis of the ferrule, but be properly guidedand constrained as well. This minimizes the repositioning caused by theadhesive flow and the thermally induced change in the separationdistance between two pairs of the input and reflective fibers. Thecapillaries provide precision parallel positioning inside the ferruleand bonding of the fibers and thereby provide a reliable constraint ofthe fibers. Preferably, the fibers touch the nearest adjacent fiber orhave a gap between the fibers of not more than about 0.5 μm. This helpsto fix the position of the fibers. It is also preferred that the fibersdo not twist around each other over the first 10 to 15 mm before thefibers enter the ferrule to reduce stress and/or fiber repositioning. Anillustrative assembly process includes the following steps. The fibersare stripped of the protective coating and cleaned for a length of about5 cm of the fiber end. The fibers are dipped into adhesive (e.g. Epo-Tek353 ND). The stripped fiber ends are then fed through the capillaryuntil the fiber coatings just reach into the cone end portion of theferrule. Additional adhesive is applied to the fibers if needed and theadhesive is allowed to wick through the entire capillary. An adhesivesuch as 353 ND adhesive with viscosity (at room temperature) of about3000 cPs (centipoise), or other suitable adhesive, can be used. Thepredicted gaps in the capillaries shown correspond to this viscosity. Ahigher viscosity adhesive (5000 to 10000 cPs) may be used if the gapsare slightly larger. An increase in temperature when inserting thefibers inside the capillaries decreases the viscosity of the adhesive.Thus having various viscosities and temperatures we can provide a betterpositioning of the fibers and minimize their repositioning after cure.In general, a suitable viscosity can be determined using theHagen-Poiseuille equation modeling viscous flow in a capillary withoptical fibers positioned in the capillary.

[0088] The assembly is cured, an 8-degree angle is polished into theferrule and anti-reflective coating is applied. The bond layers betweenthe fibers and surrounding ferrule are extremely thin (preferably lessthan about 1-1.5 μm) to minimize thermal stress and movement. Variousembodiments of the ferrule capillaries of the present invention areillustrated in FIGS. 13A to 13H and 14A to 14E.

[0089]FIG. 13A shows a cross sectional view of a ferrule 16 with arounded square or rounded rectangular capillary 130 and closely packedoptical fibers 131 a, 131 b, 131 c, and 131 d. The rounded squarecapillary provides a fixed SD, while the rounded rectangle capillary canmake the SD variable. The rounded corners and closely packed fibers makethis a good design for several reasons. The shape of the capillary alongwith the closely spaced fibers 131 effectively prevents movement of thefibers 131 prior to curing and also reduces thermal stress on the fibersafter curing. The curvature of rounded corners 130 a preferably has asmaller radius than the outer surface of fibers 131. More preferably,the corners 130 a are 90-degree angles and thus form a true square orrectangle capillary. Therefore, for purposes of this specification,“substantially rectangular” refers to a capillary cross section wherethe radius of the corners is less than or equal to the radius of theenclosed optical fibers. Gap G4 is where the fiber comes closest totouching, or actually touches, the wall of capillary 130. Gap G4 ispreferably less than about 0.5 μm, and more preferably less than about0.1 μm, and most preferably zero (i.e. the fiber touching the wall ofthe capillary). The gap G6 between the closely adjacent fibers 131 a and131 b (and also fibers 131 c and 131 d) is similarly small (i.e.preferably less than about 1.0 μm, 0.5 μm, or zero μm). The gap G5 isalso preferably small (i.e. less than about 1.0 μm, 0.5 μm, or zero μm)however, the gap G5 between the distant adjacent fibers 131 a and 131 dmay be larger to achieve a desired SD as illustrated in the followingfigures. The closely packed fibers also provide a secondary advantage inthat only a small amount of adhesive is required in the capillary 130and therefore less thermal stress is exerted on the fibers 131 due tothe unequal coefficient of thermal expansion (CTE) between the fibersand the adhesive. Even the adhesive in the larger gap G5 has been foundto have minimal effect in causing stress or shifting of the opticalfibers due to thermal expansion and contraction. This capillary designtends to prevent shifting of the fibers and prevents rotation of thefibers due to the flow of adhesive prior to cure (e.g. fiber 131 d isunlikely to rotate to the position of fiber 131 a, and fiber 131 a isunlikely to rotate to position 131 b, etc.,)

[0090] Once the fibers are affixed in the capillary 130, the selectionof which optical fibers will form pairs (i. e., input and reflective)may be made. Generally, pairs of fibers will be positioned diagonallyfrom one another. For example, referring to FIG. 13A, diagonally spacedfibers (e.g., 131 a and 131 c) may be selected for pairing. Lightsignals moving through diagonally spaced fibers may intersect at thesame point at the center of the optical filter 24. This may cause someinterference between signals. If signal interference is a problem, thenusing the capillary designs with both fixed and variable SD designs forthe fiber pairs may reduce the interference. Several capillaryconfigurations are possible and are discussed next.

[0091] Several other exemplary capillary designs include the dual-ovalcapillary (FIG. 13B), the clover-leaf or four-circular capillary (FIG.13C), the six-fiber rectangular capillary (FIG. 13D), the twowafer-ferrule (FIG. 13E and 13F), the four-fiber rectangular capillary(FIG. 13J), the dual rectangular capillary (FIG. 13K), the variable dualrectangle capillary (FIG. 13L), the dual oval capillary (FIG. 13M), themixed capillary (FIG. 13N) and the alignment washer design (FIGS. 14A &B). For simplicity, the same reference numbers are used forcorresponding features in each of the Figures.

[0092] A significant difference between the capillary designs is thatsome are useful for a “fixed” separation distance design while othersare useful for a “variable” separation distance design. For example,FIGS. 13A through 13D illustrate fixed SD designs (i.e. the SD cannot bechanged). However, FIGS. 13E through 13H illustrate variable SD designs.Generally, the variable SD designs are used when larger separationdistances are desired.

[0093] Referring now to FIG. 13B, the shape of dual-oval capillary 132resembles two adjacent ovals and the capillary 132 encloses the opticalfibers 131. Portions of capillary 132 form a constraining arc 132 a ofapproximately 120° to 180° around fibers 131. The gap G4 between thesurface of the fibers 131 and the proximate wall of the capillary 132 ispreferably less than about 1.5 μm, and more preferably less then about1.0 μm, and most preferably less than about 0.5 μm. Similarly, the gapbetween closely adjacent fibers G6 is also preferably less then about1.5 μm, and more preferably less then 1.0 μm, and most preferably lessthen about 0.5 μm at the closest point. The gap G5 between the variablydistant adjacent fibers G5 preferably ranges from 0.5 μm to about 300 μmdepending on the position of the two oval capillaries. The diagonalpairs, such as fibers 131 a and 131 c, are formed into pairs of inputand reflective optical fibers. The dual-oval capillary may be expandedto three or even four adjacent ovals if desired to form multi-ovalcapillaries. However, in the multi-oval capillaries, diagonal pairs ofoptical fibers are preferable.

[0094]FIG. 13C illustrates a four-circular capillary 133 enclosingfibers 131. Portions of capillary 133 form a constraining arc 133 a ofapproximately 180° to 240° around fibers 131. The gap G4 between thefiber and the proximate wall of the capillary is preferably less thanabout 1.5 μm, and more preferably less then about 1.0 μm, and mostpreferably less than about 0.5 μm. Also, the gap G6 between closelyadjacent fibers is similarly preferably less than about 1.5 μm, and morepreferably less then about 1.0 μm, and most preferably less than about0.5 μm.

[0095]FIG. 13D illustrates a rectangular capillary 130 enclosing sixfibers 131. Again, the gaps, G4, G5, and G6 are preferably as small aspossible to prevent movement of the fibers. The gaps are thereforepreferably less than about 1.5 μm, and more preferably less then about1.0 μm, and most preferably less than about 0.5 μm. In this embodimentthe fibers have two separation distances. The diagonal fiber pairs (i.e.131 a, 131 c and 131 b, 131 d) have matching separation distance.However, the fiber pair, 131 e and 131 f, have a smaller separationdistance. While this configuration may be of less use with thin filmfilter assemblies, this configuration is useful for certain crystalbased assemblies such as isolators.

[0096] The ferrule and capillary designs described above are examples offixed separation distance capillaries. The separation distance betweenthe fibers is fixed and cannot be changed. However, it is desirable tobe able to change or vary the separation distance. For example, if athin film filter has a certain preferred angle of incidence, then it isuseful to vary the separation distance of the fibers to correspond tothe desired AOI. The following ferrule and capillary designs provide amethod of achieving this desired separation distance while maintainingthe same positioning accuracy of the prior designs. Generally, thesedesigns maintain a fixed vertical separation between fibers whilevarying the horizontal (as seen in the Figures) distance. It has beenfound that the ability to vary the horizontal distance in a range offrom 5 μm to 75 μm is most useful.

[0097] One embodiment for a variable SD ferrule and capillary isillustrated in the two-wafer capillaries shown in FIG. 13 E where across-sectional view of four fibers 131 (two pairs) are positionedinside of V-shaped capillaries 134 a and 134 b formed from matchinggrooves in two elongated silicon plates (wafers) 135 a and 135 b. Thesilicon wafers are etched with the V-grooves and accuracy of 0.5 μm ispossible. Crystallographic orientation provides excellent anglereproducibility. Further, the wafers are easily mass produced usingcurrent etching techniques. The wafers 135 are each provided with four,preferably symmetrical, grooves. The two center grooves (i.e. fibergrooves) are used to form capillaries 134 a and 134 b when the wafersare mated together. A feature of this design is that the V-shapedgrooves may be positioned as desired to achieve any required separationdistance between the fibers 131. The adjacent fibers in each capillary134 preferably touch each other. Adhesive is applied to the gaps tosecure the fibers 131 in place. Alignment grooves in wafers form twoalignment capillaries 136 which are for aligning the wafers 135.Preferably, glass balls or rods 137 of about 300 μm diameter areinserted into alignment capillaries 136 of having dimensions of suitablesize to contain rods 137 up to about 302 μm in diameter to maintainalignment. The rods 137 preferably have dimensional tolerance of 2.0 μm,and more preferably have a tolerance of 1.0 μm, and most preferably havea tolerance of 0.5 μm. If the rods are too large, the fibers may haveexcess room to move relative to their respective grooves. The glassrods, therefore, are preferably prescreened to verify dimensionaltolerances. UV-curable tacking adhesives and thermally curablestructural adhesives are applied for providing structural integrity ofthe assembly. A more preferable wafer ferrule is illustrated in FIGS.13F through 13H.

[0098] The wafers in FIG. 13F use smaller V-shaped grooves 138 forsupporting the fibers 131 and alignment rods/pins 137. The smallerV-shaped grooves prevent the wafers from coming into contact. It isthought that this design will allow the fibers to touch adjacent fibersand thereby prevent movement or repositioning of the fibers 131. In thisembodiment, the large V-shaped grooves (i.e. alignment grooves) 138 asupport the alignment pins 137 and the smaller V-shaped grooves (i.e.fiber grooves) 138 b support the fibers 131. The large V-grooves 138 apreferably are 246 μm at their widest point. The smaller V-grooves 138 bare preferably 120 μm at their widest point. Using this design, theV-grooves that support the fibers 131 can be positioned as desired tovary the separation distance of the fibers 131. Using known etchingtechniques, the V-grooves can be positioned with a tolerance of about0.2 μm. This design is easily expanded to more fibers by merely etchingmore V-grooves for more fibers. Even though the wafers do not touch, thechannels formed by the matching grooves are still referred to ascapillaries for this specification.

[0099] The aligned and bonded wafer ferrule 16 may then be cut, etched,or machined (e.g., polished) to a polygonal or cylindrical shape orother shape as desired so that ferrule may be inserted inside aprotective glass sleeve 14. This is illustrated in FIG. 13 G. Theend-face surface is processed the same as other ferrules, the end-faceis ground to an 8° angle, polished, and coated with an anti-reflective(AR) material. One skilled in the art will understand from theseexamples that there are other similar capillary designs which willsimilarly support the positioning of optical fibers with tolerances ofabout 0.5 μm.

[0100] Generally over etching of the V-grooves is not a problem. If theV-grooves are over etched, only a uniform vertical shift in the wafersis induced. Of course, if the V-grooves are etched excessively, thefibers and alignment pins may have room to move or reposition. FIG. 13Hillustrates the relative position of fibers and alignment pins andV-grooves. The V-groove on the left easily restrains the movement of thefiber. However, the V-groove on the right side provides very littlerestraint on the fiber and is therefore less desirable.

[0101] While the wafer ferrule design has several advantages, the wafersand alignment rods can be expensive to manufacture and the process ofaligning the fibers properly into the V-grooves can be time consuming. Atechnique to reduce the disadvantages while still taking advantage ofthe high accuracy of the V-grooves will now be shown. Using this method,a convention ferrule and capillary may be used in combination withwafers to achieve a high degree of accuracy in positioning the fibers ata low cost. The process is as follows and is illustrated in FIG. 131. Aplurality of optical fibers 131 are inserted into a ferrule 16. Thefibers 131 are sufficiently long to extend out the end of the ferrule16. Two silicon wafers are etched with V-grooves in the same manner asdiscussed above. The two wafers 139 are positioned around the fibers 131such that the fibers 131 are accurately positioned in the V-grooves asdiscussed above. The wafers 139 are clamped together with a spring clampor similar device. The fibers 131 are now accurately positioned andadhesive is applied to hold the fibers in place. Using this technique,an inexpensive ferrule with a low tolerance capillary can be made toposition fibers in a very high degree of accuracy which rivals thetwo-wafer designs discussed above.

[0102] The preferred method of applying adhesive to all capillariesincludes applying small portions of adhesive 144 a and 144 b to thefibers 131 just outside of the ferrule 16 to block the flow ofsubsequently applied liquid adhesive. This adhesive is cured beforeapplying additional adhesive. Additional adhesive 144 c is applied tothe fibers and the end of the ferrule 16 and allowed to wick through thecapillary 130. The liquid adhesive is drawn through the capillary 130presumably via the process of capillary action and emerges out the endof the ferrule where it is blocked by cured adhesive 144 b. The adhesive144 c is cured and the wafers 139 are removed. The fibers 131 andferrule 16 may then be cut and polished as desired.

[0103] Another technique for applying adhesive to the fibers prior toinserting into the ferrule. This technique has the advantage that thefibers are held together by the liquid adhesive by capillary action. Theliquid adhesive may be applied by dipping the fibers into the adhesive,or preferably by applying a small amount of adhesive to the fibers.

[0104] Another design for achieving variable separation distance isillustrated in FIG. 13J. In this design a rectangular capillary 130supports four fibers 13 1. The fibers are positioned against the wallsof the capillary 130 and therefore the separation distance is controlledby the width of the capillary 130. The gaps, G4 and G6, are preferablyless than about 1.5 μm, and more preferably less then about 1.0 μm, andmost preferably less than about 0.5 μm. However, the horizontal gap G5between fibers may be as wide as desired. In other words, gap G5 is theshortest or minimum distance between the cladding of adjacent fibers 131b and 131 c.

[0105] Yet another design is the dual-rectangle capillary illustrated inFIG. 13K. The capillaries 130 may be manufactured to tolerances of lessthan 1.0 μm using currently known techniques and therefore theseparation distance between the fibers can be closely controlled. Thedimensions of the capillaries 130 specified to be 2.0 μm wider andtaller than the dimensions of the fibers 131. The tolerance for thecapillaries 130 is 2.0 μm. Therefore, there is room for inserting thefibers into the capillaries and while limiting the repositioning of thefibers.

[0106] Still yet another embodiment is illustrated in FIG. 13L. Thisembodiment allows variable positioning of the fibers 131 in both thehorizontal and the vertical positions as seen in the figure. Thisembodiment is similar to FIG. 13K in both design and tolerances.Although the design in FIG. 13L can be used to achieve large separationdistances between the fibers 131, the fibers can more easily berepositioned within the capillaries 130 due to stresses such as adhesivecuring and thermal changes. It should be noted that some care must betaken to provide a reasonable separation between the capillaries 130. Ithas been found that small separations lead to fractures and breaks inthe glass between the capillaries. In this embodiment, gap G6 is theshortest or minimum distance between the surface of the cladding of theadjacent fibers 131 c and 131 d.

[0107]FIG. 13M illustrates another dual capillary design similar to thedesign of FIG. 13K. However, in this instance the capillaries are ovalsinstead of rectangles. The same fabrication techniques and tolerancesapply to this embodiment.

[0108] A hybrid of both fixed and variable separation distance fibers isillustrated in FIG. 13N. This hybrid design incorporates features of thevarious designs discussed above. An advantage of this design is thelarge number of fibers (for example, 8 as shown in the illustratedembodiment) that are fit into a single ferrule. However, the separationdistance for the fibers is not equal. The four fibers 131 a-131 d in themiddle capillary have a small separation distance while the outer fibers131 e-131 h have larger separation distance. In this embodiment it ispreferred that the optical fibers are paired as follows: fiber 131 awith 131 c; fiber 131 b with 131 d; fiber 131 e with 131 g; and fiber131 f with 131 h. Because of the two different separation distances,this design is generally not preferred for use with thin-film filters.This design is suitable for isolators and other optical elements whichare not sensitive to AOI.

[0109] Yet another process and apparatus for positioning optical fibersinside of a ferrule uses alignment washers to precisely position thefibers. This process is illustrated in FIGS. 14A and B. The process usesalignment washers 140 shown in FIG. 14A. Washer 140 is shown having fourapertures 141 for receiving optical fibers, however it is easilyscalable to larger numbers of optical fibers. Alignment washer 140allows precision fiber placement into a ferrule 16 using simple andhighly manufacturable components. Photolithography technology may beused to manufacture the washers 140 with the precisely positionedapertures 141 and spacing between them. The diameter of apertures ispreferably about 126 μm which provides approximately 0.5 μm gap betweenthe fiber and the wall of the aperture. The tolerances for the locationof the apertures are also preferably less than about 1.0 μm and morepreferably less than about 0.5 μm for each pair of the input andreflective fibers. For example, the tolerance for the distance “D4”between the apertures 141 d and 141 b is preferably 0.5 μm. The same isapplicable to the distance “D5” between apertures 141 a and 141 c.However, the tolerance for the distance “D6” between adjacent aperturessuch as 141 a and 141 b is preferably less than about 1.0 μm and morepreferably less than about 0.5 μm. A photo-resistive material is used tofabricate the washers 140. Any other technique may be used to form thewasher as long as the necessary tolerances are achieved. The washers 140are used as optical fiber-guiding and constraining devices. Thecapillaries described above generally result in restricting fibermovement or shifting to less than about 0.5 μm.

[0110] Turning to FIG. 14B there is shown a cross section view of thewashers 140, fibers 142, and ferrule 16. Fibers 142 are inserted throughfirst washer 140 a, through ferrule 16, and through a second washer 140b. Ferrule 16 may have a conventional cylindrical capillary 130.However, the invention may be adapted for use with most capillariesregardless of shape. At this step of the process it may be helpful topre-heat the assembly to aid in the installation and precise placementof the fibers 142. The assembly may then be cooled to room temperatureto hold the fibers 142 in position while adhesive is applied. Washers140 are bonded to the end-faces of ferrule 16. In the case of a ferrulehaving a cone portion for receiving fibers (see FIG. 5) the washer 140is preferably bonded at the base of the cone portion where the capillary130 meets the cone portion. The ferrule capillary 130 is filled with aliquid adhesive via the gap created by the flat portion 143 of washer140 and either UV cured or thermally cured. The flat portion 143 mayalso be used to align the fibers at each end of the ferrule prior tocuring the adhesive. When both flat portions are aligned then the fibersare also aligned. The completed assembly is processed the same as aconventional ferrule; the end-face is ground to approximately an 8°angle, polished, and an AR coating is applied. Filter AOI and Fiber SDare discussed next.

[0111] For all of the fiber capillaries discussed above, it is importantto achieve accurate SD so that the SD can be accurately matched with afilter AOI as discussed in the next section. Further, when manufacturinga fiber-ferrule having multiple pairs of fibers, it is important for SDfor all of the pairs to be the approximately equal (with a tolerance ofabout 0.5 μm) since this tends to make the active alignment processeasier and more successful.

[0112] The next aspect of the invention is the relationship between thefilter angle of incidence (AOI) and the optical fiber separationdistance (SD). The tolerances for SD are precise so that light signalsare directed to within about 0.5 μm of the center of a desired opticalfiber core. It is helpful to define some terms prior to the generaldiscussion of AOI and SD.

[0113] Filter AOI is well known in the art and does not require lengthyexplanation. Generally, filter AOI is useful in tuning a filter to adesired center wavelength (CWL). Each filter is characterized accordingto its CWL and AOI. The AOI value represents the desired angle ofincidence for optimal performance of the filter. For proper operationand low insertion loss, the filter should be matched to a pair ofoptical fibers having a corresponding SD.

[0114] Separation distance (SD) is defined, for purposes of thisspecification, as the distance between the center of the optical fibercores of two optical fibers. The term generally refers to SD betweenpairs (i.e., an input fiber and a reflective fiber) of optical fibers.In the preferred embodiment of the invention, SD ranges from about 125μm to about 250 μm. This range of SD corresponds to an AOI range fromabout 2° to about 3° as discussed below (see FIG. 15).

[0115] It has been found that a precise, cost effective and stablealignment of a filter assembly 10 can be achieved by selectingcomponents having matching characteristics. For example, the componentsof a filter assembly include the fiber ferrule l 6, collimating lens 22,and filter 24. The characteristics, which need to be matched, includethe filter AOI, the collimating lens AOI, and the optical fiber SD. Theoptics of GRIN lenses are understood and manufacturing a GRIN lens tomatch a desired AOI is known in the art. Matching the filter AOI andoptical fiber SD is not as easy.

[0116] Generally, matching of filter AOI and fiber SD is done bycreating a database of measurements for the different sets of the filterchips and the ferrules. First, filters are tested and characterizedaccording to CWL and AOI. The measurements may be performed as follows.A filter is assembled into a filter assembly 10 or similar device sothat a light signal may be directed onto the filter. A light signal istransmitted into input optical fiber 18, transmitted through thecollimating lens 22 to filter 24. The output of filter 24 is monitoredand the pass frequency or CWL of the filter is determined. The angle ofthe light signal impacting relative to the filter is adjusted until thedesired output signal from filter 24 is achieved. Typically for thecommercial thin film filters the resulting AOI is between about 1.80 and3°.

[0117] While the filter 24 is at the desired AOI, the corresponding SDmay be determined by correlation with the SD data on the ferrule sets.

[0118] Repeated testing and measurement for various filters AOI yieldsan accurate database that relates filter AOI to a corresponding SD ofthe ferrule. Those skilled in the art understand that these measurementswill vary depending on the optical characteristics of a specific designof a filter assembly and therefore should be performed on the specificdevice for best results.

[0119] After the measurements are made and the database created,tolerances may be generated for matching input collimating assemblieswith filters for a given packaging tolerance accuracy. A table as shownin FIG. 15 can be generated showing the range of SD that may be matchedto a corresponding range of filter AOI. The components can becategorized and placed in labeled bins so that matching parts may bedone quickly and efficiently. As can be seen in FIG. 15, the range ofeach SD category is preferably about 3-4 μm. This tolerance of 3-4 μm issatisfactory for achieving an ultimate tolerance of 0.5 μm since thefilter 24 may be tilted a small amount to compensate for such smallvariances in SD without significantly changing the CWL of the filter.

[0120] The table may also be arranged for tighter tolerances if desired.This may be desirable in some cases where CWL must be very precise sincechanges in filter AOI effect the CWL of the filter. Ideally, the SDs foreach pair of fibers in an input assembly will be identical or withinabout 2 μm and therefore will place the input assembly into one of thepredefined categories as shown in FIG. 15.

[0121] Once the matching input collimating assembly (4-port ormultiple-port) 35 and the filter 24 are selected, they may be assembledas discussed above to form a filter assembly 10. The four-port input anddual-fiber output collimating assemblies will be aligned for a maximumtransmitted signal and then soldered with the outer sleeve 32 (FIG. 3)precisely retaining the interrelationships between these collimatingassemblies. The assembly of the complete multiple-port device 30 isdiscussed next.

[0122] Input and output collimating and filtering assemblies are affixedinside protective sleeve 32. Output fiber-ferrule collimating assembly35′ is manufactured in nearly the same way as input collimating assembly35. However, depending on the application, fewer of the fiber pigtails38 may be needed. Also, it is preferred to use an aspheric collimatinglens (which may be a molded aspheric lens) instead of a GRIN lens in theoutput collimating assembly 35′. Aspheric lenses have advantages inapplication to 6 port and higher port devices as compared to GRINlenses. First, aspheric lenses have a long working distance, defined asdistance from the front focal point to the front surface of the lens.For multiple-port devices, the input and output collimating assembliesshould have their focal points coincide in order to optimize theinsertion loss. This point should also coincide with the filter coatingsurface of the filter. For multiple-port collimating assemblies that areon the substrate side of the filter, the working distance must be largeenough, or the filter must be thin enough, so that the focal point canbe placed on the filter coating surface of the filter. If GRIN lensesalone are used only, then the filter thin films and substrate would needto be very thin (on the order of 240 times the refractive index of thesubstrate, in tim). At this thinness, the filter films and substrateswould have limitations associated with film stress and also highsusceptibility to breakage, cracking, etc. during manufacturing.Aspherical lenses have working distance on the order of 2 mm whichallows a standard filter and substrate thickness of about 1.5 mm (andlarger) to be used. Therefore, a preferred configuration includes a fourfiber ferrule, a GRIN or asphere lens, a bandpass (thin film filter)coating, a substrate, an asphere, and a dual fiber ferrule. Thefollowing configuration is also possible while still optimizinginsertion loss: a four fiber ferrule, an asphere lens, a substrate, abandpass (thin film filter) coating, a GRIN or asphere, and a dual fiberferrule.

[0123] Another advantage of aspheric lenses is the flexibility in focallength. In order to keep the angle of incidence to the filter low, alonger focal length of the lens is desirable. This is relatively easy toaccomplish with an aspheric lens. Molded asphere lenses are availablewith many different focal lengths at low cost. For GRIN lenses, to makethe focal length longer, the index profile must change, which representsa significant departure from the standard doping process. It isdifficult and costly to obtain GRIN lenses at arbitrary focal lengths.All of the above makes aspherical lenses more attractive for thisapplication.

[0124] Preferably, the output collimating assembly 35′ is manufacturedin the same way and to the same tolerances as the input collimatingassembly 35. This is preferred so that the location of output opticalfibers 38 will match with the corresponding reflective fiber 20 in theinput collimating assembly 35. Also, it is easier to determine the SDcharacteristic. If pairs of optical fibers are not used in the outputcollimating assembly 35′, then an estimate of the SD is made. The outputcollimating assembly 35′ is optically aligned with filter 24 bymicro-tilting, rotating, and axially adjusting the assembly 35′ formaximum transmission. This is possible because the interior dimension ofprotective sleeve 32 is substantially larger than the exteriordimensions of output assembly 35′. Micro-tilting may be achieved by amicro-tilting device grasping both the protective sleeve 32 and the endof the output assembly 35′ that extends from the protective sleeve 32.The preferred embodiment provides a gap of about 50-100 μm which issufficient to permit micro-tilting of output assembly 35′ inside ofsleeve 32. Once the active alignment of output collimating assembly 35′is complete, output collimating assembly 35′is affixed using a solder oradhesive 33 which is inserted into the gap between the exterior ofcollimating assembly 35′ and the protective sleeve 32.

[0125] The previous discussion has related to how to manufacturemultiple-port devices such as four-fiber ferrules and six and eight-portfiltering packages. The following discussion relates to furtherapplications of these devices and additional advantages of theinvention.

[0126] Turning first to FIG. 16A there is shown a schematic diagram of afour-port filtering assembly which includes a first input fiber 160 a, afirst reflective fiber 160 b coupled to a second input fiber 160 c, anda second reflective fiber 160 d. Also illustrated are ferrule 16, lens22, and filter 24. In operation, a light signal is input through firstinput fiber 160 a, collimated by lens 22 and partially reflected byfilter 24. The reflected signal received by first reflective fiber 160 band communicated to second input fiber 160 c. The signal is againcollimated by lens 22 and partially reflected by filter 24 and finallyreceived by second reflective fiber 160 d which can communicate thesignal to a optical communications system, network or a desireddestination. Features of this device provide enhanced performance whichis useful in optical communication systems.

[0127] First, the same filter is used to reflect the signal each time.This is an advantage over devices that required two distinct filters toperform this function. Further, the performance is enhanced because thefiltering characteristic is identical for each reflection. In devicesusing two filters, the filters typically have filtering characteristics,that are similar but not identical. Therefore, this design may result inimproved filtering characteristics such as, for example, sharper andsteeper cut-off frequencies. An example of uses for such devices is anotch filter. Typically one reflection from a thin-film notch filterwill provide 12 to 15 dB of separation. A second reflection from thesame filter will yield a separation of 24 to 30 dB. The device also hasapplication to various other shaping filters.

[0128] A second feature which improves performance is the couplingbetween the first reflective fiber 160 b and the second input fiber 160c. Both of these fibers may be formed from a single, unbroken opticalfiber. This eliminates the requirement for an optical coupling devicebetween two separate fibers. Coupling devices typically have insertionloss associated with their use. Elimination of the coupling devicetherefore improves the performance of the four-port filter 161.

[0129] Another embodiment of the four-port filter 161 is suitable forgain flattening filters commonly associated with optical amplifiers. Asillustrated in FIG. 16B, a signal is input by first input fiber 160 a,reflected by gain flattening filter 24 to first reflective fiber 160 b.The signal is amplified by amplifier 162 and communicated to secondinput fiber 160 c. The signal is again reflected by gain flatteningfilter 24 to second reflective fiber 160 d.

[0130] In another embodiment, a single filter assembly 161 may be usedto gain flatten the signals from two amplifiers 162. FIG. 16C shows aschematic view of filtering assembly 161 coupled to two amplifiers, 162a and 162 b. A light signal is input through fist input fiber 160 a andreflected by gain-flattening filter 24. The reflected signal travelsthrough first reflective fiber 160 b to first amplifier 162 a. Theamplified signal travels back to filter assembly 161 through secondinput fiber 160 c where it is again reflected by gain-flattening filter24 and output through second reflective fiber 160 d to second amplifier162 b.

[0131]FIG. 16D is an opto-mechanical schematic of a five-port filter163. The operation of the filter is very similar to the assembly of FIG.16A, however, this five-port filter includes an output collimatingassembly for receiving the signal transmitted through the filter 24.Filter 24 may be any of a variety of thin film filters, such as, fordescriptive purposes, a narrow band-pass filter. A light signal is inputby first input fiber 160 a, collimated by lens 22 and partiallyreflected by filter 24. The selected narrow band portion of the signalis transmitted through the filter 24 to transmitted fiber 160 e. Thereflected portion of the signal is communicated through first reflectivefiber 160 b and second input fiber 160 c and reflected again by filter24. The twice reflected signal is then output by second reflective fiber160 d and the isolation from the transmitted frequency is as high as24-30 dB.

[0132] In yet another embodiment, the filtering package is coupled toheat sink ports or terminals 165 to dissipate excess signal energy. Inthis embodiment, illustrated in FIG. 16E, filter 24 may be any of avariety of thin film type filters such as a band-pass filter orgain-flattening filter. A first input signal is transmitted throughfirst input fiber 160 a, collimated by lens 22 and partially reflectedand partially transmitted by filter 24. The transmitted portion istransmitted through lens 34 to first transmitted fiber 160 e which ispresumably coupled to a communications system. The reflected portion ofthe first input signal is reflected back through lens 22 to firstreflective fiber 160 b which is coupled to a first terminal 165 a.Terminals 165 are heat dissipation devices commonly known in the artwhich harmlessly dissipate the waste energy. A similar path is followedby a second light signal that is transmitted through second input fiber160 c. The transmitted portion of the signal is transmitted to secondtransmitted fiber 160 f and the reflected waste energy portion ischanneled to a second terminal 165 b via second reflective fiber 160 d.

[0133]FIG. 16F illustrates another embodiment with integrated wasteenergy heat sink ports. However, in this embodiment the heat sinkterminals 165 are coupled to the transmitted fibers 160 e and 160 f. Thereflected signals are output through first and second reflective fibers160 b and 160 d and presumably connected to a communications system.

[0134]FIG. 16G is a schematic diagram of an add/drop package using theinstant invention. Filter 24 is a thin-film band pass filter whichpasses light at wavelength λ1 and reflects all other wavelengths. Lenses22 and 34 are preferably collimating GRIN lenses. A first light signalenters via input fiber I₁ with wavelengths λ₁ . . . λ_(n). Wavelength λ₁is transmitted to fiber T₁ and wavelengths λ₂ . . . λ_(n) are reflectedto fiber R₁, and thereby one channel or wavelength is dropped orde-multiplexed from the input signal. Conversely, another signal isinput to fiber I₂ with wavelengths λ₂ . . . λ_(n) and a third signal isinput to fiber I₃ with a wavelength of λ₁. The wavelengths λ₂ . . .λ_(n) are reflected by filter 24 to fiber R₂. In addition, thewavelength λ₁ is transmitted through filter 24 to fiber R₂. Thereforefiber R₂ exits the package 171 with wavelengths λ₁ . . . λ_(n) andthereby one channel or wavelength is added or multiplexed into theoriginal signal from fiber I₂.

[0135]FIG. 16H is a schematic diagram of an eight-port package 166. Thisembodiment includes four input fibers I₁ . . . I₄ that are coupled tofour transmitted fibers T₁ . . . T₄ through an input collimating lens22, an optical element 24, and an output collimating lens 34. Theoptical element may be any of various shaping filters such as again-flattening filter or band-pass filter. However, this embodiment isparticularly well-suited to be used with a crystal element, such as anisolator, as the optical element of choice.

[0136] The final embodiment of an optical package is an eight-portadd/drop device. FIG. 16I is a schematic diagram of the package which iscapable of both adding and dropping a channel for two separate lightsignals. The operation is as follow. A first light signal withwavelengths λ₁ . . . λ_(n) is input through fiber I₁. Filter 24 is aband pass filter which passes only light of wavelength λ₁ and reflectsall other wavelengths. Therefore, wavelength λ₁ is transmitted to fiberT₁ and the remaining wavelengths, λ₂ . . . λ_(n), are reflected to fiberR₁. A second signal having a wavelength λ₁ is input through fiber I₂.Fiber I₂ is optically aligned with fiber R₁ and therefore the signal ispassed through filter 24 and coupled to fiber R₁ and the resultingsignal on fiber R₁ contains wavelengths λ₁ . . . λ_(n). Thus, theoriginal channel at wavelength λ₁ and a new channel at wavelength λ₁ hasbeen added. The same operation is accomplished on fibers I₃, I₄, R₂, andT₂.

[0137] In yet another embodiment, a compact DWDM module is created asschematically illustrated in FIG. 17. This figure illustrates afour-channel add/drop module useful in a communications system.Concatenating four six-port filtering packages 171 (such as described inrelation to FIG. 16G) together creates the module. Beginning with thede-multiplex (i.e. drop function), the demux signal containingwavelengths λ₁ . . . λ_(n) enters the package via first input fiber I₁of package 171 a and is collimated by input lens 22. A portion of thesignal, λ₁, is transmitted through filter 24 a and transmitted out ofthe module via transmitted fiber T₁. The remaining wavelengths λ₂ . . .λ_(n) are reflected to reflective fiber R₁ and communicated to the firstinput fiber of package 171 b. Filter 24 b in package 171 b transmitswavelength λ2 to transmitted fiber T₂ and reflects the remainingwavelengths λ₃ . . . λ_(n) to reflective fiber R₂ which communicates thesignal to the first input fiber of package 171 c. The process continuesand wavelength λ₃ is transmitted to fiber T₃ in package 171 c.Similarly, wavelength λ₄ is transmitted to fiber T₄ in package 171 d.

[0138] The DWDM module 170 also multiplexes signals. Starting with thesecond input fiber I2 of package 171 a, a signal of wavelength λ₁ istransmitted through filter 24 a to transmitted fiber T5 and coupled topackage 171 b. In package 171 b a signal of wavelength λ₂ is similarlyinput and transmitted through filter 24 b. Filter 24 b reflectswavelength λ₁ and thereby causes both wavelengths to be multiplex andcommunicated to package 171 c. In package 171 c, wavelength λ₃ is addedto wavelengths λ₁ and λ₂. The signal containing all three wavelengths iscommunicated to package 171 d where wavelength λ₄ is added.

[0139] It will become apparent to those skilled in the art that variousmodifications to the preferred embodiment of the invention as describedherein can be made without departing from the spirit or scope of theinvention as defined by the appended claims.

The invention claimed is:
 1. A method of making a multiple-port opticalfiltering device comprising the steps of: providing an optical filtercharacterized according to AOI of the filter; providing an inputcollimating assembly comprising a plurality of pairs of optical fibers,each of said pairs comprising an input fiber and a reflective fiber,said assembly characterized as a function of separation distances (SD)of said pairs of input and reflective fibers disposed in said assembly,said collimating assembly satisfying a predetermined tolerance formatching said separation distances and said AOI of said filter; mountingsaid filter in a filter-holding unit, said filter-holding unitcomprising a cavity for mounting to said input collimating assembly,said cavity having a sufficiently large diameter to permit micro-tiltingof said filter-holding unit relative to said collimating assembly; andoptically aligning said filter and said collimating assembly such thatinsertion loss is within predetermined limits.
 2. The method of claim 1,further comprising the step of affixing said filter-holding unitrelative to said input collimating assembly to maintain the opticalalignment.
 3. The method of claim 1, wherein said input collimatingassembly includes a fiber ferrule comprising a capillary design suitableto maintain separation distance of said optical fibers within about 0.5μm tolerance limit.
 4. The method of claim 3, wherein a gap between eachof said optical fibers and a proximate wall of the capillary of saidferrule is less than about 1.5 μm.
 5. The method of claim 4, wherein agap between each of said optical fibers and an adjacent optical fiber isless than about 1 μm.
 6. The method of claim 1, wherein said inputcollimating assembly comprises a ferrule having at least one capillary,and wherein said capillary is a type selected from the group consistingof a rounded square, a rounded rectangle, a dual-oval, and afour-circular capillary.
 7. The method of claim 6, the tolerance for thedimensions of said at least one capillary is less than about 1.0 μm. 8.The method of claim 1, wherein said input collimating assembly comprisesa ferrule having at least one capillary, and wherein said capillary isformed from two wafers each comprising grooves such that when the wafersare properly aligned at least one capillary is formed by matchinggrooves in each wafer.
 9. The method of claim 1, wherein said inputcollimating assembly comprises a ferrule having at least one capillary,and wherein said ferrule comprises optical fibers passing through saidcapillary and said fibers being aligned by at least one alignment washersecured to said ferrule.
 10. The method of claim 1, wherein the step ofoptically aligning includes an iterative alignment process ofrepetitively monitoring the amplitude of at least one reflected lightsignals and changing the angle of the filter relative to an incominglight signal.
 11. The method of claim 10, wherein the step of opticallyaligning includes an iterative alignment process comprising moving saidfilter holding unit in at least two degrees of freedom.
 12. The methodof claim 11, wherein the step of optically aligning includes aniterative alignment process comprising moving said filter holding unitin at least three degrees of freedom.
 13. The method of claim 1, furthercomprising the steps of: providing an output collimating assemblycomprising at least one optical fiber; and optically aligning saidoutput collimating assembly with said optical filter such that insertionloss is reduced below a predetermined level.
 14. The method of claim 13,further comprising the steps of: providing a protective sleeve forenclosing said filter-holding unit and said output collimating assembly,said protective sleeve having a sufficiently large interior to allow formicro-tilting of said output collimating assembly relative to saidfilter-holding unit; positioning said optical filter and said outputcollimating assembly at least partially inside of said sleeve; andsecuring said filter-holding unit and said output collimating assemblyto the interior of said protective sleeve.
 15. The method of claim 14,wherein the step of providing a protective sleeve includes a protectivesleeve having interior dimensions sufficiently large to provide a gapbetween the exterior of at least one of the collimating assemblies andthe interior of the protective sleeve, and wherein said gap between theexterior of at least one of the collimating assembly and the interior ofthe protective sleeve is about 50-100 pm.
 16. A method of fabricating amultiple-port optical device comprising the steps of: providing anoptical element holder comprising a cavity for mounting to a collimatingassembly, said cavity having sufficiently large dimensions to permitmicrotilting of said holder relative to said collimating assembly;mounting an optical element to said optical element holder, said opticalelement characterized according to a desired AOI; selecting an inputcollimating assembly comprising a plurality of pairs of optical fibersoptically coupled with a collimating element, said assemblycharacterized as a function of the separation distance of said pairs ofoptical fibers, and wherein said collimating assembly is selected as afunction of said AOI of said optical element; optically aligning saidoptical element and said input collimating assembly to satisfy apredetermined maximum insertion loss; and securing said optical elementrelative to said input collimating assembly to preserve the opticalalignment.
 17. The method of claim 16, wherein said optical element isselected from the group consisting of an optical filter, a circulator,an isolator, an attenuator, and a crystal.
 18. The method of claim 16,wherein said pairs of optical fibers include input fibers and reflectivefibers, and wherein the step of optically aligning includes iterativelymonitoring output signals from said reflective fibers and adjusting theposition of said optical element holder to achieve a predeterminedinsertion loss.
 19. The method of claim 18, wherein the step ofoptically aligning includes adjusting said optical element holder in atleast two degrees of freedom.
 20. The method of claim 19, wherein thestep of optically aligning includes adjusting said optical elementholder in at least three degrees of freedom.
 21. The method of claim 20,wherein the step of optically aligning includes adjusting said opticalelement holder in at least four degrees of freedom.
 22. The method ofclaim 21, wherein the step of optically aligning includes adjusting saidoptical element holder in at least five degrees of freedom.
 23. Themethod of claim 16, wherein said input collimating assembly comprises afiber ferrule comprising at least one capillary, and wherein said fiberferrule is selected from the group consisting of: a ferrule having arounded rectangle capillary, a ferrule having a rounded squarecapillary, a ferrule having a dual-oval capillary, a ferrule having afour-circular capillary, a two-wafer ferrule comprising capillariesformed from grooves, and a ferrule comprising a washer for positioningoptical fibers.
 24. The method of claim 23, wherein the gap between thefibers and the proximate wall of the capillary is less than about 1.5μm.
 25. The method of claim 23, wherein the gap between adjacentproximate fibers is less than about 1.5 μm.
 26. The method of claim 23,wherein the capillaries maintain the separation distances between thefibers to within a tolerance of about 0.5 μm.
 27. The method of claim16, further comprising the steps of: providing an output collimatingassembly; providing a protective sleeve suitable for enclosing saidoptical element holder and said output collimating assembly, said sleevecomprising interior dimensions sufficiently larger than the exteriordimensions of said output collimating assembly as to permit microtiltingof said output collimating assembly relative to said sleeve; insertingsaid output collimating assembly and said optical element into saidsleeve; and actively aligning said output collimating assembly with saidfilter by iteratively monitoring output signals from output opticalfibers and moving said output collimating assembly relative to saidfilter to minimize insertion loss.
 28. A multiple-port, temperaturecompensated, optical filtering package comprising: an optical filtercharacterized according to desired AOI; an input collimator assemblycomprising, a ferrule assembly comprising at least one capillaryenclosing at least two pairs of optical fibers, said ferrule assemblycharacterized as a function of separation distances of said pairs ofoptical fibers, said ferrule having a SD characteristic corresponding tosaid AOI, said ferrule having an input end and an output end; and acollimating lens having an input end and an output end, said input endpositioned to receive light signals from said ferrule and said outputend of said lens communicating light signals to said filter; and afilter-holder supporting said optical filter and comprising a cavity forreceiving said output end of said collimating lens, the size of saidcavity sufficient to permit microtilting of said filter-holder relativeto said collimating lens.
 29. The multiple-port, temperaturecompensated, optical filtering package of claim 28, further comprising,an output collimating assembly; a protective sleeve enclosing saidfilter-holder, said input collimating assembly, and said outputcollimating assembly; and wherein the interior of said protective sleeveis sufficiently large to permit micro-tilting of at least one of saidcollimating assemblies relative to said filter.
 30. The multiple-port,temperature compensated, optical filtering package of claim 29, whereinsaid output collimating assembly comprises an aspheric collimating lens.31. The multiple-port, temperature compensated, optical filteringpackage of claim 29, wherein the interior of said protective sleeve isof a size to allow a gap of about 50-100 μm between the exterior of atleast one of said collimating assemblies and the interior of saidprotective sleeve.
 32. The multiple-port, temperature compensated,optical filtering package of claim 28, wherein the type of capillary isselected from the group consisting of a rounded square capillary, arounded rectangle capillary, a dual-oval capillary, a four-circularcapillary, capillaries formed from symmetrical grooves in two-wafers,and capillaries with at least one washer.
 33. The multiple-port,temperature compensated, optical filtering package of claim 32, whereina gap between each of said optical fibers and the proximate wall of thecapillary is less than about 1.5 μm.
 34. The multiple-port, temperaturecompensated, optical filtering package of claim 33, wherein a gapbetween each of said optical fibers and an adjacent fiber is less thanabout 1.0 μm.